The invention relates to a photocathode comprising on a transparent support successively a glass bonding layer, a passivating layer and a photo-emissive layer.
Such a photocathode is known from French patent No. 2,300,413.
One of the problems occurring in the manufacture of such photocathodes relates to the connection of the photo-emissive layer to the support. During said connection the crystal properties of the photo-emissive layer on which the future photo-emission properties depend should not be spoiled or disturbed.
Another problem is that as soon as the connection has been made the formed assembly must be capable of withstanding a thermal treatment at high temperature which precedes the activation of the photo-emissive layer so as to make said layer photo-emissive by the supply of, for example, caesium, without the crystal properties of the semiconductor being disturbed.
A second problem relates to the connection of the support to an envelope of a tube. The support must give the photocathode a good mechanical rigidity. Therefore it must be thick. Moreover the support is connected with its edge to the envelope of a tube. Hence said edge must project sufficiently beyond the surface of the photo-emissive layer so that welding to the envelope does not spoil the photo-emissive layer. As a result of the large thickness and the large area the support has a rather large volume. Therefore, in order not to augment the costs of the tube it is desirable that the material of the support should not be too expensive.
U.S. patent No. 3,769,536 suggests making said support from glass, the photo-emissive layer being bonded to the glass at a suitable temperature. Although the use of glass has the advantage of being cheap, it also has disadvantages. Also, when a glass is used the properties of thermal expansion and the melting and softening points which have been adapted to those of the photo-emissive layer in such manner that welding is facilitated thereby, the required temperatures are disadvantageous for the crystal properties of the semiconductor material of the photo-emissive layer. This is probably because during welding the glass of the support is brought to a high temperature and very large stresses are formed in the glass during cooling. The thermal treatment of the assembly obtained after welding prior to the caesium treatment of the semiconductor, for the glass, is comparable with the beginning of a firing treatment. The stresses of the glass are liberated and introduce other stresses in the semiconductor material so that the crystal structure is disturbed.
In French patent No. 2,300,413, the photo-emissive layer is sealed on a support of monocrystalline oxide (for example, corundum) by means of a bonding glass, the coefficients of expansion of the photo-emissive layer, of the bonding glass and of the oxide of the support being mutually adapted, the transition and softening temperatures of the oxide being much higher than those of the bonding glass and of the photo-emissive layer. During said melting operation the monocrystalline oxide of the support is heated to temperatures which are much lower than those of the transition point thereof in such manner that after said melting no stresses are formed upon cooling which may be liberated during the later thermal treatment. During the bonding operation and thermal treatment the monocrystalline oxide to a certain extent plays the role of stiffener which results in preserving the crystalline properties of the photo-emissive layer. Furthermore, the good thermal conductivity of the oxide is very attractive during the thermal treatment of the photo-emissive layer after sealing. To the contrary, said monocrystalline oxide, irrespective of its nature, always is an expensive material which increases the price of the photocathode and hence the electron tube.